Various alloys may be characterized as being “crack sensitive”. Ingots and other workpieces composed of crack sensitive alloys may form cracks along their surfaces and/or edges during hot working operations. Forming articles from crack sensitive alloys may be problematic because, for example, cracks formed during forging or other hot working operations may need to be ground off or otherwise removed, increasing production time and expense, and reducing yield.
During certain hot working operations, such as forging and extrusion, dies apply a force to an alloy workpiece to deform the alloy workpiece. The interaction between the die's surfaces and the alloy workpiece's surfaces may involve heat transfer, friction, and wear. One conventional technique for reducing surface and edge cracking during hot working is to enclose the alloy workpiece in a metallic canister before hot working. With a cylindrical workpiece, for example, the inside diameter of the metallic canister may be slightly larger than the outside diameter of the alloy workpiece. The alloy workpiece may be inserted into the metallic canister such that the metallic canister loosely surrounds, but is not metallurgically bonded to, the alloy workpiece. The dies may contact the outer surfaces of the metallic canister. The metallic canister thermally insulates and mechanically protects the enclosed alloy workpiece, thereby eliminating or reducing the incidence of crack formation on the alloy workpiece. The metallic canister thermally insulates the alloy workpiece by action of the air gaps between the alloy workpiece and the metallic canister's inner surfaces, and also by directly inhibiting the alloy workpiece from radiating heat to the environment.
An alloy workpiece canning operation may result in various disadvantages. For example, mechanical contact between forging dies and the metallic canister's outer surfaces may break apart the metallic canister. Also, during upset-and-draw forging of a canned workpiece, the metallic canister may break apart during the draw operation, and in such case the alloy workpiece may need to be re-canned between each upset-and-draw cycle of a multiple upset-and-draw forging operation. Re-canning increases process complexity and expense. Further, a metallic canister may impair an operator from visually monitoring the surface of a canned alloy workpiece for cracks and other work-induced defects.
Given the foregoing drawbacks, it would be advantageous to provide a more efficient and/or more cost-effective method of hot working crack sensitive alloys. More generally, it would be advantageous to provide a method for improving the hot workability of alloy ingots and other alloy workpieces.